1. Field of the Invention
The present invention relates to a technique for holding various substrates including a semiconductor substrate, a printed circuit board, a substrate for a color filter, a glass substrate for a flat panel display provided in a liquid crystal display device and a plasma display device, a substrate for an optical disk, a solar cell panel and the like (hereinafter referred to simply as “substrates”).
2. Description of the Background Art
The technique of holding a substrate is essential for various substrate processing apparatuses (as disclosed in, for example, Japanese Patent Application Laid-Open No. 5-150175 (1993)) which perform various processes on a substrate (for example, the process of irradiating a photosensitive material formed on a substrate with light, the process of applying various coating solutions onto a substrate, and the like) and for substrate transport apparatuses which transport a substrate.
In regard to this technique, for example, Japanese Patent Application Laid-Open Nos. 2008-270626 and 2005-19637 each disclose a structure which uses vacuum suction nozzles to hold the lower surface of a substrate under suction. Also, for example, Japanese Patent Application Laid-Open Nos. 2009-28863, 2005-142462, 2002-64130 and 2010-52051 each disclose a structure which uses Bernoulli nozzles based on Bernoulli's principle (or nozzles which eject a gas from holes formed in a holding surface to attract a substrate by suction using the Bernoulli effect) to hold a substrate under suction. Also, for example, Japanese Patent Application Laid-Open No. 2004-193195 discloses a structure which uses vacuum suction nozzles to fix a substrate so as to prevent the substrate from moving while using Bernoulli nozzles to lift the substrate. Also, for example, Japanese Patent Application Laid-Open No. 2006-80289 discloses a structure which uses both Bernoulli nozzles and vacuum suction nozzles to hold a substrate under suction.
In a drawing apparatus and the like which directs light onto a photosensitive material formed on a substrate to form a pattern of a circuit and the like on the substrate, drawing accuracy is lowered if the substrate is out of position relative to a holding surface during processing. It is hence necessary to securely hold the substrate placed on the holding surface so that the substrate is prevented from being out of position.
A substrate placed on a holding surface is not necessarily flat in shape. For example, when a hand of a transport apparatus with a substrate placed on the holding surface thereof supports a portion of the substrate near the center thereof, there is a possibility that the substrate placed on the holding surface is warped in a convex form. When the hand of the transport apparatus supports an outer edge of the substrate, there is a possibility that the substrate placed on the holding surface is warped in a concave form. In recent years in which substrates become thinner, the substrates have been much prone to be warped. Thus, there is a high possibility that the substrates placed on the holding surface are warped.
In the structures configured to hold the substrate under suction on the holding surface by forming a negative pressure between the substrate and the holding surface as disclosed in the aforementioned cited references, it is very difficult to securely hold a warped substrate. This is because the negative pressure is relieved through a gap which is created between part of the back surface of the substrate and the holding surface if the substrate placed on the holding surface is warped. Unless a sufficient suction pressure is formed between the holding surface and the back surface of the substrate, suction failure occurs to increase the possibility that the substrate is out of position relative to the holding surface. On the other hand, when an attempt is made to securely hold the substrate under suction without the occurrence of such suction failure, it is necessary that a very large sucking force is exerted on the substrate. To this end, there arises a need to provide, for example, a large-sized vacuum pump and a large-scale piping system, which inevitably results in the increase in the size and costs of the apparatus.